<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>阴影</title>
</head>
<body>
    <div class="shadow"></div>
</body>
<style>
    :root {
        --size: 100px;
    }
    .shadow {
        width: var(--size);
        height: var(--size);
        box-shadow: inset -10px 0 10px 10px rgba( 25, 25, 25, 0.4);
    }
</style>
</html>